Multi-level shielded multi-conductor interconnect bus for MEMS

ABSTRACT

A multi-level shielded multi-conductor interconnect bus for use in interconnecting MEM devices with control signal sources and a method of fabricating a multi-level shielded multi-conductor interconnect bus are disclosed. In one embodiment, a multi-level shielded interconnect bus ( 410 A) formed on a substrate ( 20 ) includes first and second level electrically conductive lines ( 42, 92 ) arranged in sets of one, two or more conductive lines between first and second level electrically conductive shield walls ( 46, 66, 96 ). The first and second level electrically conductive lines ( 42, 92 ) are surrounded by various layers of dielectric material ( 30, 50, 80, 100 ). A first level electrically conductive shield ( 78 ) overlies the first level electrically conductive lines ( 42 ) and shield walls ( 46, 66 ). A second level electrically conductive shield ( 112 ) overlies the second level electrically conductive lines ( 92 ) and shield walls ( 96 ).

RELATED APPLICATION INFORMATION

This application is a divisional of and claims priority from U.S. patentapplication Ser. No. 10/099,720 entitled “MULTI-LEVEL SHIELDEDMULTI-CONDUCTOR INTERCONNECT BUS FOR MEMS” filed on Mar. 15, 2002, theentire disclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to microelectromechanicalsystems (MEMS), and more particularly to the design and fabrication ofinterconnect architectures for MEMS.

BACKGROUND OF THE INVENTION

MEMS can include numerous electromechanical devices fabricated on asingle substrate, many of which are to be separately actuated in orderto achieve a desired operation. For example, a MEMS optical switch mayinclude numerous mirrors that are each positionable in a desiredorientation for reflecting optical signals between originating andtarget locations upon actuation of one or more microactuators associatedwith each mirror. In order for each mirror to be separately positioned,separate control signals need to be supplied to the microactuatorsassociated with each mirror. One manner of accomplishing this is toconnect each microactuator to a control signal source with a separateelectrical conductor (i.e., an interconnect line) fabricated on thesurface of the substrate that extends between its associatedmicroactuator and a bond pad at the periphery of the substrate where itcan be easily connected to an off-chip control signal source. In thisregard, the separate interconnect lines together comprise aninterconnect bus and are typically arranged to run parallel with eachother for substantial portions of their length.

As may be appreciated, the amount of footprint required on the surfaceof the substrate for an interconnect bus is an important factor indesigning MEMS since increasing the footprint of the interconnect busdecreases the amount of footprint available for desired devices (e.g.,mirrors and actuators). Another consideration is possible cross-talkbetween the separate interconnect lines. Cross-talk is a problem becausea control signal intended for one actuator can be coupled from itsinterconnect line into adjacent interconnect lines causing undesiredactuation of other actuators. A further consideration is the possibilityof shorting between adjacent interconnect lines. Where the interconnectbus lines are exposed on the surface of the substrate, particles and thelike can settle across adjacent interconnect lines thereby causing shortcircuits effecting operation of the MEMS.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a shielded multi-conductorinterconnect bus for MEMS and a method for fabricating such aninterconnect bus. The shielded multi-conductor interconnect bus of thepresent invention substantially reduces the possibility of cross-talkbetween adjacent interconnect lines, alleviates the possibility of shortcircuits due to particles and the like settling across adjacentinterconnect lines, and optimizes the amount of footprint required forsuch an interconnect bus.

According to a first aspect of the present invention, a multi-levelshielded multi-conductor interconnect bus is provided having first andsecond levels of shielded electrically conductive lines. The secondlevel electrically conductive lines may be oriented parallel with thefirst level electrically conductive lines or they may be orientedtransverse to the first level electrically conductive lines. Themulti-level shielded multi-conductor interconnect bus includes asubstrate. The substrate may, for example, be comprised of silicon. Afirst dielectric layer overlies and is supported by at least a portionof the substrate. In this regard, the first dielectric layer may, forexample, be the lowest layer of material on the substrate (i.e., it maybe formed directly on the upper surface of the substrate without anyintervening layers). In one embodiment, the substrate is comprised ofsilicon and the first dielectric layer comprises a dielectric stackdeposited directly on the upper surface of the substrate that includes alower layer of thermal oxide and an upper layer of silicon nitride. Aplurality of substantially parallel first level electrically conductivelines are formed on the first dielectric layer. A first levelelectrically conductive shield is formed in a spaced relation above thefirst level electrically conductive lines. A plurality of first levelelectrically conductive walls are formed on the first dielectric layer.Although desirable, it should be understood that electrically conductivewalls described herein do not have to be continuous along theirlengthwise extent and may, in fact, have one or more breaks formedtherein as desired. The first level electrically conductive wallstypically extend parallel with the first level electrically conductivelines and include upper sections in contact along at least a portionthereof with a lower side of the first level electrically conductiveshield. The first level electrically conductive lines and the firstlevel electrically conductive walls are arranged in pattern such that atleast one first level electrically conductive wall is located betweensets of the first level electrically conductive lines, with each set offirst level electrically conductive lines including at least one firstlevel electrically conductive line.

The first dielectric layer may also include a plurality of firstchannels formed therein with lower sections of the first levelelectrically conductive walls being formed in the first channels. Wherethe first dielectric layer is the lowest layer of material on thesubstrate, each first channel preferably extends vertically downwardthrough the entire thickness of the first dielectric layer along atleast a portion of each first channel, and, more preferably along theentire length of each first channel to permit the lower sections of thefirst level electrically conductive walls to contact the upper surfaceof the substrate.

The multi-level shielded multi-conductor interconnect bus also includesa plurality of substantially parallel second level electricallyconductive lines formed in a spaced relation above the first levelelectrically conductive shield. A second level electrically conductiveshield is formed in a spaced relation above the second levelelectrically conductive lines. A plurality of second level electricallyconductive walls are formed above the first level electricallyconductive shield. The second level electrically conductive wallstypically extend parallel with the second level electrically conductivelines and include lower sections in contact along at least a portionthereof with an upper side of the first level electrically conductiveshield and upper sections in contact along at least a portion thereofwith a lower side of the second level electrically conductive shield.The second level electrically conductive lines and second levelelectrically conductive walls are arranged in pattern such that at leastone of the second level electrically conductive walls is located betweensets of second level electrically conductive lines, with each set ofsecond level electrically conductive lines including at least one secondlevel electrically conductive line.

In one embodiment, the first level electrically conductive lines and thelower sections of the first level electrically conductive walls areformed from a first layer of doped polysilicon, the upper sections ofthe first level electrically conductive walls and the first levelelectrically conductive shield are formed from a second layer of dopedpolysilicon (which may be comprised of a thinner lower layer of dopedpolysilicon and a thicker upper layer of doped polysilicon), the secondlevel electrically conductive lines and the lower sections of the secondlevel electrically conductive walls are formed from a third layer ofdoped polysilicon, and the second level electrically shield and theupper sections of the second level electrically conductive walls areformed from a fourth layer of doped polysilicon.

The first and second level electrically conductive lines may besurrounded by dielectric material. In this regard, the multi-levelshielded multi-conductor interconnect bus may also include second, thirdand fourth layers of dielectric material (e.g., silicon dioxide orsilicate glass). The second dielectric layer overlies the first levelelectrically conductive lines and first dielectric layer and includes aplurality of channels formed therein permitting the upper sections ofthe first level electrically conductive walls to extend verticallyupward therethrough to contact the first level electrically conductiveshield. The third dielectric layer overlies the first level electricallyconductive shield and includes a plurality of channels formed thereinpermitting the lower sections of the second level electricallyconductive walls to extend vertically downward therethrough to contactthe first level electrically conductive shield. The fourth dielectriclayer overlies the second level electrically conductive lines and thirddielectric layer and includes a plurality of channels formed thereinpermitting the upper sections of the second level electricallyconductive walls to extend vertically upward therethrough to contact thesecond level electrically conductive shield. Where there is nodielectric material around the second level electrically conductivelines, there may be a plurality of anchor posts supporting the secondlevel electrically conductive lines in the spaced relation above thefirst level electrically conductive shield.

It should be noted that a multi-level shielded multi-conductorinterconnect bus in accordance with the present invention may befabricated on a substrate that has one or more intervening layers ofelectrically conductive material and/or dielectric material between theupper surface of the substrate and the first layer of dielectricmaterial. In this regard, the channels in the first dielectric layerextend vertically down into the first dielectric layer to expose theupper surface of an intervening layer of electrically conductivematerial, and the lower sections of the first level electricallyconductive walls contact the intervening layer of electricallyconductive material, rather than the substrate.

According to another aspect of the present invention, a three-levelshielded multi-conductor interconnect bus includes a substrate andfirst, second, third, and fourth layers of electrically conductivematerial deposited over and supported by at least a portion of thesubstrate. A plurality of first level electrically conductive lines areformed in the first layer of electrically conductive material, aplurality of first level electrically conductive walls are formed in thefirst layer of electrically conductive material, a plurality of secondlevel electrically conductive lines are formed in the second layer ofelectrically conductive material, a plurality of second levelelectrically conductive walls are formed in the second layer ofelectrically conductive material, a plurality of third levelelectrically conductive lines are formed in the third layer ofelectrically conductive material, a plurality of third levelelectrically conductive walls are formed in the third layer ofelectrically conductive material, and an electrically conductive shieldis formed in the fourth layer of electrically conductive material. Inthis regard, the first, second and third level electrically conductivelines and the first, second and third level electrically conductivewalls may be substantially parallel with one another. The layers ofelectrically conductive material may, for example, be comprised of dopedpolysilicon. In one embodiment, the second layer of electricallyconductive material is comprised of two separately deposited layers ofdoped polysilicon.

The three-level shielded multi-conductor interconnect bus may alsoinclude a first layer of dielectric material (e.g. thermal oxide andsilicon nitride) and second, third and fourth layers of dielectricmaterial (e.g., silicon dioxide or silicate glass) deposited over andsupported by at least a portion of the substrate. The first layer ofdielectric material is disposed between the first level electricallyconductive lines and the substrate, the second layer of dielectricmaterial is disposed between the second level electrically conductivelines and the first level electrically conductive lines, the third layerof dielectric material is disposed between the third level electricallyconductive lines and the second level electrically conductive lines, andthe fourth layer of dielectric material is disposed between theelectrically conductive shield and the third level electricallyconductive lines.

According to a further aspect of the present invention, a method formaking a multi-level shielded multi-conductor interconnect bus beginswith removing portions of a first layer of dielectric material overlyingand supported by at least a portion of a substrate to provide aplurality of substantially parallel first channels in the first layer ofdielectric material. In this regard, where the first layer of dielectricmaterial comprises a dielectric stack on an upper surface of thesubstrate, sufficient material may be removed so that the first channelsextend vertically downward through the entirety of the dielectric stackto expose the upper surface of the substrate preferably along at least aportion of each first channel, and, more preferably, along the entirelength of each first channel. A first layer of electrically conductivematerial is then deposited over the first layer of dielectric material,with the first layer of electrically conductive material also fillingthe first channels. Strips of the first layer of electrically conductivematerial are then removed to provide a plurality of first levelelectrically conductive lines typically extending parallel with thefirst channels. In this regard, an upper surface of the first layer ofdielectric material is exposed at the bottom of each strip removed fromthe first layer of electrically conductive material. A second layer ofdielectric material is then deposited over the first layer ofelectrically conductive material, with the second layer of dielectricmaterial also filling the strips removed from the first layer ofelectrically conductive material. Portions of the second layer ofdielectric material are removed to provide a plurality of substantiallyparallel second channels in the second layer of dielectric material. Thesecond channels are located to overlie and are oriented in the samedirection as the first channels and extend downward through the secondlayer of dielectric material to expose the first layer of electricallyconductive material filling the first channels. A second layer ofelectrically conductive material is deposited over the second layer ofdielectric material, with the second layer of electrically conductivematerial filling in the second channels. In one embodiment, the step ofdepositing a second layer of electrically conductive material maycomprise the steps of depositing a lower layer of doped polysilicon,depositing an intervening layer of sacrificial material, removing theintervening layer of sacrificial material, and depositing an upper layerof doped polysilicon.

After the second layer of electrically conductive material is deposited,a third layer of dielectric material is deposited over the second layerof electrically conductive material. Portions of the third layer ofdielectric material are removed to provide a plurality of substantiallyparallel third channels in the third layer of dielectric material. Inthis regard, the third channels extend downward through the third layerof dielectric material to expose the second layer of electricallyconductive material at the bottom of each third channel. A third layerof electrically conductive material is deposited over the third layer ofdielectric material, with the third layer of electrically conductivematerial also filling the third channels. Strips of the third layer ofelectrically conductive material are removed to provide a plurality ofsecond level electrically conductive lines typically extending parallelwith the third channels. In this regard, an upper surface of the thirdlayer of dielectric material is exposed at the bottom of each stripremoved from the third layer of electrically conductive material. Afourth layer of dielectric material is deposited over the third layer ofelectrically conductive material, with the fourth layer of dielectricmaterial also filling the strips removed from the third layer ofelectrically conductive material. Portions of the fourth layer ofdielectric material are removed to provide a plurality of substantiallyparallel fourth channels in the fourth layer of dielectric material. Thefourth channels are located to overlie and are oriented in the samedirection as the third channels and extend downward through the fourthlayer of dielectric material to expose the third layer of electricallyconductive material filling the third channels. The multi-levelmulti-conductor interconnect bus is completed by depositing a fourthlayer of electrically conductive material over the fourth layer ofdielectric material, with the fourth layer of electrically conductivematerial also filling the fourth channels.

These and other aspects and advantages of the present invention will beapparent upon review of the following Detailed Description when taken inconjunction with the accompanying figures.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and furtheradvantages thereof, reference is now made to the following DetailedDescription, taken in conjunction with the drawings, in which:

FIGS. 1A-1E are cross-sectional views illustrating the microfabricationprocess of a portion of one embodiment of a shielded multi-conductorinterconnect bus in accordance with the present invention;

FIGS. 2A-2B are top views illustrating the microfabrication process of aportion of the shielded multi-conductor interconnect bus shown in FIGS.1A-1E;

FIGS. 3A-3B are perspective cross-sectional views of portions of twoembodiments of a shielded multi-conductor interconnect bus in accordancewith the present invention having enhanced conductive line density;

FIG. 3C is a perspective cross-sectional view of a portion of anembodiment of a shielded multi-conductor interconnect bus in accordancewith the present invention having a single shielded electricallyconductive line breaking out from the bus;

FIG. 3D is a top cross-sectional view of the portion of the shieldedmulti-conductor interconnect bus having a single shielded electricallyconductive line breaking out from the bus;

FIG. 3E is a cross-sectional view of the single shielded electricallyconductive line;

FIGS. 4A-4B are perspective cross-sectional and cross-sectional views ofa portion of one embodiment of a multi-level shielded multi-conductorinterconnect bus in accordance with the present invention;

FIGS. 4C-4D are perspective cross-sectional and cross-sectional views ofa portion of another embodiment of a multi-level shieldedmulti-conductor interconnect bus in accordance with the presentinvention;

FIG. 5A is a perspective cross-sectional view of a portion of anembodiment of a multi-level shielded multi-conductor interconnect bus inaccordance with the present invention having staggered conductive linessupported by anchor posts;

FIGS. 5B-5C are top cross-sectional views of portions of two embodimentsof a multi-level shielded multi-conductor interconnect bus in accordancewith the present invention having non-staggered conductive linessupported by anchor posts;

FIG. 6 is a cross-sectional view of an embodiment of a multi-levelshielded multi-conductor interconnect bus in accordance with the presentinvention having three levels of conductive lines; and

FIG. 7 is a top view of an embodiment of a multi-level shieldedmulti-conductor interconnect bus in accordance with the presentinvention where upper level conductive lines shift to the lower level.

DETAILED DESCRIPTION

Referring to FIGS. 1A-1E and FIGS. 2A-2B, there are showncross-sectional and top views, respectively, illustrating themicrofabrication process of a portion of one embodiment of a shieldedmulti-conductor interconnect bus 10. The microfabrication process beginswith a substrate 20 having a first dielectric layer 30 formed thereon.In this regard, the substrate 20 may be comprised of silicon, and thefirst dielectric layer 30 may be comprised of a lower layer 30A ofthermal oxide (e.g., typically about 630 nanometers thick) formed by awet oxidation process at an elevated temperature (e.g., 1050° C. forabout 1.5 hours) and an upper layer 30B of silicon nitride (e.g.,typically about 800 nanometers thick) deposited over the thermal oxidelayer using a low-pressure chemical vapor deposition (LPCVD) process ata temperature of about 850° C.

A plurality of substantially parallel channels 32 are formed in thefirst dielectric layer 30. The channels 32, as with other features ofthe shielded multi-conductor interconnect bus 10 wherein material isremoved from one or more layers of material, may be formed, for example,by a mask and etch removal process employing appropriate masking agentsand etchants depending upon the material that is to be removed. Eachchannel 32 preferably extends vertically down through the firstdielectric layer 30 to expose the upper surface of the substrate 20 inone or more locations along the length of the channel 32, and morepreferably, along the entire length of the channel 32. FIG. 1A shows across-sectional view and FIG. 2A shows a top view after the channels 32have been formed in the first dielectric layer 30.

After the channels 32 in the first dielectric layer 30 are formed, afirst layer of an electrically conductive material (the firstelectrically conductive layer 40) is deposited over the remainingportions of the first dielectric layer 30 and in the channels 32. Thefirst electrically conductive layer 40 is comprised of an electricallyconductive material such as, for example, polycrystalline silicon (alsotermed polysilicon). In this regard, the first electrically conductivelayer 40 is also referred to herein as the Poly0 layer 40. The Poly0layer is typically about 300 nanometers thick with subsequentpolysilicon layers being thicker (e.g., typically between about 1.0 and2.5 microns thick). The Poly0 layer 40 (and other polysilicon layersdescribed hereafter) may be deposited using a LPCVD process at atemperature of about 580° C. In depositing the Poly0 layer (and otherpolysilicon layers described hereafter), various dopant materials (e.g.,phosphorous) can be employed to make the polysilicon electricallyconductive.

After the Poly0 layer 40 is deposited, a plurality of parallel Poly0conductive lines 42 are formed between the filled channels 32 in thefirst dielectric layer 30 by removing strips 44 from the Poly0 layer 40on both sides of each channel 32 in the first dielectric layer 30. ThePoly0 conductive lines 42 are electrically isolated from the substrate20 along their entire length by the first dielectric layer 30 underneaththe Poly0 conductive lines 42. The Poly0 material remaining in and abovethe filled channels 32 forms Poly0 shield walls 46 between each of thePoly0 conductive lines 42. The strips 44 removed from the Poly0 layer 40extend down to the first dielectric layer 30 along their entire lengthin order to electrically isolate the Poly0 conductive lines 42 from thePoly0 shield walls 46. The strips 44 may be located at a small distancefrom the sides of each channel 32 in the first dielectric layer 30 sothat the Poly0 shield walls 46 overlap the first dielectric layer 30 bya small amount on either side of each channel 32. This small overlapallows for alignment tolerance during the fabrication process andensures that the Poly0 shield wall 46 completely seals and protects thelower oxide layer 30A. FIG. 1B shows a cross-sectional view and FIG. 2Bshows a top view after the strips 44 have been removed from the Poly0layer 40 in order to form the Poly0 conductive lines 42 and Poly0 shieldwalls 46.

After the Poly0 conductive lines 42 and Poly0 shield walls 46 are formedin the Poly0 layer 40, a second dielectric layer 50 is deposited overthe Poly0 layer 40. The second dielectric layer 50 is comprised of anelectrically insulating material such as, for example, a sacrificialmaterial (e.g., silicon dioxide or silicate glass). In this regard, thesecond dielectric layer 50 is also referred to herein as the Sacox1layer 50. The Sacox1layer 50 (and other sacrificial layers describedherein) may be deposited using a LPCVD process at a temperature of about580° C. The Sacox1 layer 50 (and subsequent sacrificial layers) istypically about 2.0 microns thick. The Sacox1 layer 50 fills in thestrips 44 removed from the Poly0 layer 40. A plurality of channels 52are then formed in the Sacox1 layer 50. Each of the channels 52 in theSacox1 layer 50 is located and oriented to coincide with a correspondingone of the channels 32 in the first dielectric layer 30 and extends downthrough the Sacox1 layer 50 to expose the upper surface of the Poly0shield 46 formed in its corresponding channel 32 in the first dielectriclayer 30. The upper surface of each Poly0 shield wall 46 is exposed bythe channel 52 in the Sacox1 layer 50 in one or more locations along itslength, and preferably is exposed along the entire length of the Poly0shield wall 46. FIG. 1D shows a cross-sectional view after the channels52 have been formed in the Sacox1 layer 50 to expose the upper surfacesof the Poly0 shield walls 46.

After the channels 52 are formed in the Sacox1 layer 50, a second layerof electrically conductive material (the second electrically conductivelayer 60) is deposited. The second electrically conductive layer 60 iscomprised of an electrically conductive material such as, for example,doped polysilicon. In this regard, the second electrically conductivelayer 60 is also referred to herein as the Poly1 layer 60. The Poly1layer 60 fills the bottom and sidewalls of the channels 52 in the Sacox1layer 50 and covers the remaining portions of the Sacox1 layer 50.

A third layer of electrically conductive material (the thirdelectrically conductive layer 70) is then deposited over the Poly1 layer60. The third electrically conductive layer 70 is comprised of anelectrically conductive material such as, for example, dopedpolysilicon. In this regard, the third electrically conductive layer 70is also referred to herein as the Poly2 layer 70. Prior to depositingthe Poly2 layer 70 over the Poly1 layer 60, a third dielectric layer(not shown) of sacrificial material may have been deposited over thePoly1 layer 60 and removed from the regions of the Poly1 layer 60 ofinterest to the structures described herein. The third dielectric layer(the Sacox2 layer) may be utilized in maintaining desired separationbetween the Poly1 and Poly2 layers 60, 70 in othermicroelectromechanical structures, but such separation is not desiredherein. In this regard, the Poly1 and Poly2 layers 60, 70 may beconsidered to be a single layer of polysilicon material. The Poly2 layer70 fills in the remainder of the channels 52 made in the Sacox1 layer 50to form, together with the Poly1 layer 60, Poly1/Poly2 shield walls 66on top of the Poly0 shield walls 46 and a horizontal Poly1/Poly2 shield78 over the Poly0 conductive lines 42.

The Poly1/Poly2 shield 78 is electrically connected to the substrate 20by the Poly0 and Poly1/Poly2 shield walls 46, 66 formed in the channels32, 52 in first dielectric layer 30 and the Sacox1 layer 50 on eitherside of each Poly0 conductive line 42. Thus, each Poly0 conductive line42 is, in effect, surrounded along its lengthwise extent by dielectricmaterial that is in turn encased in an equipotential, electricallyconductive tube thereby keeping the various Poly0 conductive lines 42electrically isolated from one another. Additionally, the Poly1/Poly2shield 78 also prevents shorting between Poly0 conductive lines 42 bypreventing particles or the like from contacting adjacent Poly0conductive lines 42, as might happen in interconnect buses where theconductive lines are exposed along their lengthwise extent.

It should be noted that in the figures described herein, the variouspolysilicon and sacrificial layers and structures shown are idealizedrepresentations of the actual layers and structures that are formed inthe various processing steps. In this regard, the corners of variousstructures (e.g., the channels 32, 52 and strips 44) may be somewhatrounded as opposed to square as is depicted, and layers of materialoverlying the channels 32, 52 and strips 44 may, for example, havedepressions coinciding with the locations of the channels 32, 52 andstrips 44 instead of being perfectly level across the channels 32. Thesize of the depressions and other defects, if any, may be reducedthrough the use of intermediate chemical mechanical polishing steps toplanarize the various layers of polysilicon and sacrificial materialafter they are deposited.

Multiple Conductors

Referring now to FIGS. 3A-3B, it is possible to increase the density ofthe shielded multi-conductor interconnect bus 10 while maintaining thesame width Poly0 conductive lines 42 and Poly0 and Poly1/Poly2 shieldwalls 46, 66. Enhanced Poly0 conductive line 42 density is desirable inorder to reduce the amount of footprint required for the interconnectbus and thus increase the amount of footprint available for thefabrication of MEM devices on substrate 20. When the possibility ofcross-talk between some of the Poly0 conductive lines 42 is not asignificant concern, enhanced density may be achieved by grouping setsof Poly0 conductive lines 42 together between Poly0 and Poly1/Poly2shield walls 46, 66. FIG. 3A shows a cross-sectional view of a portionof a second embodiment of a shielded multi-conductor interconnect bus310A wherein the Poly0 conductive lines 42 are grouped into sets havingtwo conductive lines 42 in each set. The Poly0 and Poly1/Poly2 shieldwalls 46, 66 are located between the sets of Poly0 conductive lines 42to reduce or eliminate possible cross-talk between the sets ofconductive lines 42. Such a shielded multi-conductor interconnect bus310A is particularly suited for feeding control signals to MEM mirrorpositioning systems having two MEM actuators for each positionablemirror because cross-talk between the pair of Poly0 conductive lines 42interconnecting the pair of actuators associated with each mirror may beof limited concern. It should be noted that in other embodiments, eachset of Poly0 conductive lines 42 need not have the same number of Poly0conductive lines 42. For example, as is illustrated in thecross-sectional view of FIG. 3B, some sets of Poly0 conductive lines 42may have only one Poly0 conductive line 42, other sets may have twoPoly0 conductive lines 42, and other sets may have three or more Poly0conductive lines 42.

In the previously described embodiments of the shielded multi-conductorinterconnect bus 10, 310A-B, it has been assumed that the Sacox1 layer50 remains over the Poly0 conductive lines 42. In some cases, the Sacox1layer 50 may be removed in part or in its entirety (e.g., duringsubsequent etching of additional layers). In such situations, thepossibility that the Poly1/Poly2 shield 78 might come into contact withthe Poly0 conductive lines 42 due to electrostatic attractive forces orcapillary forces resulting from wet chemical processing pulling thePoly1/Poly2 shield 78 downward thereby causing a short circuit situationneeds to be considered. One manner of alleviating this possibility is tolimit the lateral spacing between the Poly0 and Poly1/Poly2 shield walls46, 66 that support the Poly1/Poly2 shield 78 above the Poly0 conductivelines 42. The required lateral spacing depends upon a number of factors,including the flexibility of the Poly1/Poly2 shield 78 and theanticipated voltage difference(s) between the Poly1/Poly2 shield 78 andthe Poly0 conductive lines 42. In this regard, the Poly0 and Poly1/Poly2shield walls 46, 66 are preferably laterally spaced no more than 10 to20 microns apart, although several times this distance (e.g., 50microns) is possible under the right conditions. This assumes that thePoly1/Poly2 shield is approximately 2.5 microns thick and theanticipated voltage difference between the Poly0 conductive lines andthe Poly1/Poly2 shield is less than 300V.

Referring now to FIGS. 3C-3D, there is shown a portion of an embodimentof a shielded multi-conductor interconnect bus 310C having multiplePoly0 conductive lines 42 grouped into sets between the Poly0 andPoly1/Poly2 shield walls 46, 66, with one of the Poly0 conductive lines42 breaking away from the bus 310C. In this regard, outer Poly0conductive line 42A may, for example, be broken away from the bus 310Cto connect it to a bond pad, MEM actuator, or other MEM structure towhich Poly0 conductive line 42A feeds electrical signals. FIG. 3E showsa cross-sectional view of the single shielded Poly0 conductive line 42Athat is broken out of the bus 310C.

In order to allow Poly0 conductive line 42A to break transversely awayfrom the bus 310C, there is a break formed in the outer Poly0 andPoly1/Poly2 shield walls 46A, 66A. To maintain the shielding around thetransversely oriented Poly0 conductive line 42A, the outer Poly0 andPoly1/Poly2 shield walls 46A, 66A are continued alongside Poly0conductive line 42A. The Poly1/Poly2 shield 78 is likewise continuedover the Poly0 conductive line 42A, supported in a spaced relation abovethe Poly0 conductive line 42A by the Poly0 and Poly1/Poly2 shield walls46, 66. It will be appreciated that conductive lines may break away fromany of the shielded interconnect buses described herein in a similarmanner. Further, it will be appreciated that multiple conductive linesmay break away from a shielded interconnect bus, either individually oras a group between one pair of conductive shield walls.

Multiple Level Interconnects

Referring now to FIGS. 4A-4D, in addition to grouping conductive linesinto sets, the density of conductive lines can also be increased byadding additional layers of conductive lines. FIGS. 4A-4B showperspective cross-sectional and cross-sectional views of a portion ofone embodiment of a multi-level shielded multi-conductor interconnectbus 410A that has two levels of conductive lines. The multi-levelshielded multi-conductor interconnect bus 410A includes a fourthdielectric layer 80 overlying the Poly2 layer 70. The fourth dielectriclayer 80 is comprised of an electrically insulating material such as,for example, a sacrificial material (e.g. silicon dioxide or silicateglass). In this regard, the fourth dielectric layer 80 is also referredto herein as the Sacox3 layer 80. Parallel channels 82 are formed in theSacox3 layer 80. The channels 82 in the Sacox3 layer 80 extendvertically down through the Sacox3 layer to expose the upper surface ofthe Poly2 layer 70 in one or more locations along the length of thechannels 82, and preferably expose the upper surface of the Poly2 layer70 along the entire length of each channel 82.

A fourth layer of an electrically conductive material (the fourthelectrically conductive layer 90) is formed over the Sacox3 layer 80.The electrically conductive material comprising the fourth electricallyconductive layer 90 is, for example, doped polysilicon. In this regard,the fourth electrically conductive layer 90 is also referred to hereinas the Poly3 layer 90. The Poly3 layer 90 fills in the channels 82 inthe Sacox3 layer 80. Poly3 conductive lines 92 and shield walls 96 areprovided by removing strips 94 from the Poly3 layer 90 on each side ofthe channels 82 in the Sacox3 layer 80. A fifth dielectric layer 100comprised of for example, a sacrificial material (e.g. silicon dioxideor silicate glass), is formed over the Poly3 conductive lines 92 andshield walls 96. The fifth dielectric layer 100 is also referred toherein as the Sacox4 layer 100. Channels 102 aligned over the Poly3shield walls 96 are formed in the Sacox4 layer 100 to expose the uppersurfaces of the Poly3 shield walls 96 along at least portions of, andpreferably their entire, length. A fifth electrically conductive layer110 (also referred to herein as the Poly4 layer 110), comprised of, forexample, doped polysilicon is deposited over the Sacox4 layer 100 andinto the channels 102 in the Sacox4 layer 100 to provide a horizontalPoly4 shield 118 over the Poly3 conductive lines.

In the multi-level shielded multi-conductor interconnect bus 410A ofFIGS. 4A-4B, the upper level Poly3 conductive lines 92 and shield walls96 are oriented in the same direction as the lower level Poly0conductive lines 42 and shield walls 46. However, it is also possible toorient the upper level Poly3 conductive lines 92 and shield walls 96transverse to the lower level Poly0 conductive lines 42 and shield walls46. FIGS. 4C-4D show perspective cross-sectional and cross-sectionalviews of a portion of a second embodiment of a multi-level shieldedmulti-conductor interconnect bus 410B that has two levels of conductivelines 42, 92, with the upper level conductive lines 92 being orientedtransverse to the lower level conductive lines 42.

Although other microfabrication processes may be employed in fabricatingmulti-level shielded multi-conductor interconnect buses 410A-D asdescribed above, the SUMMiT V™ surface micromachining process developedat Sandia National Laboratories and described, for example, in U.S. Pat.No. 6,082,208, issued Jul. 4, 2000 entitled “Method For FabricatingFive-Level Microelectromechanical Structures And MicroelectromechanicalTransmission Formed”, incorporated by reference herein, is particularlyuseful for fabricating the multi-level shielded multi-conductorinterconnect buses 410A-D. Employing the SUMMiT V™ surfacemicromachining process to fabricate the multi-level shieldedmulti-conductor interconnect buses 410A-D permits easy incorporation ofthe interconnect buses 410A-D into MEM systems fabricated from fivepolysilicon levels such as some MEM mirror positioning systems useful inoptical cross connects and the like.

Referring now to FIGS. 5A-5C, in the previously described multi-levelshielded multi-conductor interconnect buses 410A-D, the Sacox3 andSacox4 layers 80, 100 remain around the Poly3 conductive lines 92 tosupport the second level Poly3 conductive lines 92 above the Poly2 layer70 and prevent electrostatic or capillary attractive forces from pullingthe Poly3 conductive lines 92 downward into contact with the Poly2 layer70, upward into contact with the Poly4 layer 110, or sideways intocontact with the Poly3 shield walls 96 on either side thereof. However,it is possible to fabricate multi-level shielded multi-conductorinterconnect buses where the Sacox3 and Sacox4 layers 80, 100 areremoved (e.g. during subsequent etching steps) from around the Poly3conductive lines 92, either partially or in their entirety. In thisregard, etch release holes (not shown) or the like may be formed in thevarious polysilicon layers 40, 70, 90, 110 in order to allow for theremoval of isolated or encapsulated sacrificial material where desired.The possibility of undesirable contact between the Poly3 conductivelines 92 and either the Poly2 layer 70, the Poly3 shield walls 96, orthe Poly4 layer 110 can be alleviated by periodically anchoring thePoly3 conductive lines 92 along their length.

FIG. 5A shows an embodiment of a multi-level shielded multi-conductorinterconnect bus 510A wherein anchor posts 120 are periodically spacedalong the length of the Poly3 conductive lines 92. The anchor posts 120extend downward through appropriately sized holes formed in the Poly2layer 70 (and the Sacox1 layer 50 if it has not also been removed) andrest on top of the first dielectric layer 30 without contacting thelower level Poly0 conductive lines 42. In this regard, the lower levelPoly0 conductive lines 42 and upper level Poly3 conductive lines 92 arestaggered so that there is adequate space between adjacent Poly0conductive lines 42 to accommodate the anchor posts 120 therebetweenthat support each upper level Poly3 conductive line 42. In somefabrication processes (e.g., the SUMMiT V™ process), isolated pads ofPoly0 layer 40 material that are slightly larger than thecross-sectional area of the anchor posts 120 will typically befabricated beneath the anchor posts 120.

As an alternative to staggering the lower level Poly0 conductive lines42 and upper level Poly3 conductive lines 92, the Poly0 conductive lines42 can also be configured to have appropriately sized holes formedtherethrough that accommodate the anchor posts 120. By way of example,FIG. 5B shows a top cross-sectional view of a multi-level shieldedmulti-conductor interconnect bus 510B taken at the interface between thePoly0 layer 40 and the Sacox1 layer 50 wherein the Poly0 conductivelines 42 are configured to have donut-like sections in order to provideholes therethrough for the anchor posts 120. The donut-like sections ofadjacent Poly0 conductive lines 42 are staggered along the lengthwiseextent of the Poly0 conductive lines 42 in order to reduce the lateralwidth required for the interconnect bus 510B. As may be appreciated, thePoly0 conductive lines 42 may be configured in many other manners aswell in order to accommodate the anchor posts 120. For example, FIG. 5Cshows a top cross-sectional view of a multi-level shieldedmulti-conductor interconnect bus 510C taken at the interface between thePoly0 layer 40 and the Sacox1 layer 50 wherein the Poly0 conductivelines 42 are configured to have lateral jogs at various locations alongtheir lengthwise extent in order to accommodate the anchor posts 120.

Referring now to FIG. 6, there may be more than two levels of conductivelines. For example, FIG. 6 shows a multi-level shielded multi-conductorinterconnect bus 610 having three levels of conductive lines. Themulti-level shielded multi-conductor interconnect bus 610 includes Poly0conductive lines 42, Poly2 conductive lines 72, and Poly3 conductivelines 92 and a Poly4 shield 118 over the three level Poly0, Poly2, andPoly3 conductive lines 42, 72, 92. The Sacox1, Sacox3 and Sacox4 layers50, 80, 100 remain around the conductive lines 42, 72, 92 in order tosupport the conductive lines 42, 72, 92 and prevent undesired movementof the conductive lines 42, 72, 92 due to electrostatic attractiveforces. In addition to having three levels of conductive lines 42, 72,92, the conductive lines 42, 72, 92 may also be grouped at each levelinto sets between the Poly0, Poly1, Poly2 and Poly3 shield walls 46, 66,76, 96 (e.g., sets of two conductive lines 42, 72, 92 each as in shownin FIG. 6). As may be appreciated, the number of conductive lines 42,72, 92 in each set need not be the same across the same level or atdifferent levels.

Boundary Conditions

In the previously described embodiments of a multi-level shieldedmulti-conductor interconnect bus 410A-D, 510A-C, 610, it is desirable toshift the conductive lines 72, 92 in the upper levels down to the Poly0level 40 prior to where they reach their endpoints (e.g., where theycontact a bond pad at one end and an actuated structure at the other).Shifting the upper level conductive lines 72, 92 down to the Poly0 levelmay be necessary because where the conductive lines 72, 92 exit theirshielding, the Sacox1, Sacox3 and Sacox4 layers 50, 80, 100 supportingthe conductive lines 72, 92 may be etched away, and thus unless theupper level conductive lines 72, 92 are supported at their endpoints,the upper level conductive lines 72, 92 will be cantilevered over thesubstrate 20 near their endpoints and thus inherently weak.

One manner of getting the upper level conductive lines 72, 92 down tothe Poly0 level is shown in FIG. 7. In FIG. 7, there is shown a top viewof a two-level shielded multi-conductor interconnect bus 710 (with thePoly4 shield 118 represented by the dotted line box) having Poly0conductive lines 42 and Poly3 conductive lines 72, with the Poly0 andPoly3 conductive lines 42, 72 being staggered. The technique illustratedin FIG. 7 can also be applied to shift down Poly4 conductive lines 92where the conductive lines 42, 72, 92 are appropriately staggered. As isshown in FIG. 7, prior to where the Poly3 conductive lines 72 reachtheir endpoints, an anchor 120 is formed that extends between the Poly3conductive line 72 and the level of the Poly0 layer 40 where theconductive line 72 is continued at the Poly0 level to a correspondingbond pad 130 or MEM device. The anchor posts 120 that shift the Poly3conductive lines 72 down to the Poly0 layer 40 are preferably formed ata sufficient distance back from where the Poly3 conductive lines 72 exitfrom under the Poly4 shield 118 in order to ensure that enoughsacrificial material remains around the Poly3 conductive lines 72 beyondthe anchor post 120 to provide adequate support of the Poly3 conductivelines 72. In this regard, it is also possible to have a small cavitybetween where the anchor posts 120 shift the Poly3 conductive lines 72down to the Poly0 layer 40 and the end of the sacrificial material. Itshould be noted that, in addition to solving the problem of havingunsupported upper level conductive lines 72 adjacent to their endpoints,shifting the upper level conductive lines 72 down to the Poly0 level hasthe added advantage of making it simpler to attach the lower and upperlevel conductive lines 42, 72 of the interconnect bus 710 to an array ofdevices since the endpoints of all of the conductive lines 42, 72 are atthe same level.

While various embodiments of the present invention have been describedin detail, further modifications and adaptations of the invention mayoccur to those skilled in the art. However, it is to be expresslyunderstood that such modifications and adaptations are within the spiritand scope of the present invention.

1. A method for making a multi-level shielded multi-conductorinterconnect bus comprising the steps of: removing portions of a firstlayer of dielectric material overlying and supported by at least aportion of a substrate to provide a plurality of parallel first channelsin the first layer of dielectric material; depositing a first layer ofelectrically conductive material over the first layer of dielectricmaterial, the first layer of electrically conductive material alsofilling the first channels; removing strips of the first layer ofelectrically conductive material to provide a plurality of first levelelectrically conductive lines extending parallel with the firstchannels, wherein an upper surface of the first layer of dielectricmaterial is exposed at the bottom of each strip removed from the firstlayer of electrically conductive material; depositing a second layer ofdielectric material over the first layer of electrically conductivematerial, the second layer of dielectric material also filling thestrips removed from the first layer of electrically conductive material;removing portions of the second layer of dielectric material to providea plurality of parallel second channels in the second layer ofdielectric material, the second channels being located to overlie andoriented in the same direction as the first channels and extendingdownward through the second layer of dielectric material to expose thefirst layer of electrically conductive material filling the firstchannels; depositing a second layer of electrically conductive materialover the second layer of dielectric material, wherein the second layerof electrically conductive material also fills the second channels;depositing a third layer of dielectric material over the second layer ofelectrically conductive material; removing portions of the third layerof dielectric material to provide a plurality of parallel third channelsin the third layer of dielectric material, the third channels extendingdownward through the third layer of dielectric material to expose thesecond layer of electrically conductive material at the bottom of eachthird channel; depositing a third layer of electrically conductivematerial over the third layer of dielectric material, the third layer ofelectrically conductive material also filling the third channels;removing strips of the third layer of electrically conductive materialto provide a plurality of second level electrically conductive linesextending parallel with the third channels, wherein an upper surface ofthe third layer of dielectric material is exposed at the bottom of eachstrip removed from the third layer of electrically conductive material;depositing a fourth layer of dielectric material over the third layer ofelectrically conductive material, the fourth layer of dielectricmaterial also filling the strips removed from the third layer ofelectrically conductive material; removing portions of the fourth layerof dielectric material to provide a plurality of parallel fourthchannels in the fourth layer of dielectric material, the fourth channelsbeing located to overlie and oriented in the same direction as the thirdchannels and extending downward through the fourth layer of dielectricmaterial to expose the third layer of electrically conductive materialfilling the third channels; and depositing a fourth layer ofelectrically conductive material over the fourth layer of dielectricmaterial, the fourth layer of electrically conductive material alsofilling the fourth channels.
 2. The method of claim 1 wherein in saidstep of removing portions of the first layer of dielectric material, thesubstrate is comprised of silicon and the first dielectric layercomprises a dielectric stack layer covering an upper surface of thesubstrate, the dielectric stack layer comprising a lower layer ofthermal oxide and an upper layer of silicon nitride.
 3. The method ofclaim 2 wherein in said step of removing portions of the first layer ofdielectric material, sufficient material is removed to provide firstchannels in the first layer of dielectric material that extendvertically downward through the first layer of dielectric material toexpose the upper surface of the substrate along at least a portion ofeach first channel.
 4. The method of claim 1 wherein in said steps ofremoving portions of the first layer of dielectric material and removingstrips of the first layer of electrically conductive material, theportions and strips are removed in a manner providing first channels andfirst level electrically conductive lines arranged in a pattern whereinone of the first channels is located between sets of the first levelelectrically conductive lines, each set of first level electricallyconductive lines including at least one first level electricallyconductive line, and wherein in said steps of removing portions of thethird layer of dielectric material and removing strips of the thirdlayer of electrically conductive material, the portions and strips areremoved in a manner providing third channels and second levelelectrically conductive lines arranged in a pattern wherein one of thethird channels is located between sets of the second level electricallyconductive lines, each set of second level electrically conductive linesincluding at least one second level electrically conductive line.
 5. Themethod of claim 1 wherein in said steps of depositing the second, thirdand fourth layers of dielectric material, the dielectric materialcomprises one of silicon dioxide and silicate glass.
 6. The method ofclaim 1 wherein in said steps of depositing the first, second, third,and fourth layers of electrically conductive material, the electricallyconductive material comprises doped polysilicon.
 7. The method of claim1 further comprising the steps of: removing the third dielectric layerfrom beneath the second level electrically conductive lines; and forminga plurality of anchor posts supporting the second level electricallyconductive lines in a spaced relation above the third layer ofelectrically conductive material.
 8. The method of claim 1 wherein saidstep of depositing a second layer of electrically conductive materialcomprises the steps of: depositing a lower layer of doped polysilicon;depositing an intervening layer of sacrificial material; removing theintervening layer of sacrificial material; and depositing an upper layerof doped polysilicon.